A coding technology for compressing a voice signal, audio signal or the like to a low bit rate signal is particularly important from the standpoint of effectively using a transmission path capacity (channel capacity) of radio waves or the like and a recording medium in a mobile communication system.
Examples of a voice coding scheme for coding a voice signal include schemes like G726, G729 standardized by the ITU-T (International Telecommunication Union Telecommunication Standardization Sector). These schemes use narrow band signals (300 Hz to 3.4 kHz) as coding targets and can perform high quality coding at bit rates of 8 kbits/s to 32 kbits/s. However, since such a narrow band signal is so narrow that its frequency band is a maximum of 3.4 kHz, the quality thereof is such that it gives the user an impression that a sound is muffled, which results in a problem that it lacks a sense of realism.
Furthermore, there is also a voice coding scheme that uses wideband signals (50 Hz to 7 kHz) as coding targets. Typical examples of this are 6722, 6722.1 of ITU-T and AMR-WB of 3GPP (The 3rd Generation Partnership Project). These schemes can perform coding of wideband voice signals at a bit rate of 6.6 kbits/s to 64 kbits/s. However, when the signal to be coded is voice, although a wideband signal has relatively high quality, it is not sufficient when an audio signal is the target or a voice signal of higher quality with a sense of realism is required.
On the other hand, when a maximum frequency of a signal is generally on the order of 10 to 15 kHz, it is possible to obtain a sense of realism equivalent to FM radio, and when the maximum frequency is on the order of up to 20 kHz, it is possible to obtain quality comparable to that of CD (compact disk). For such a signal, audio coding represented by the layer III scheme or AAC scheme standardized by MPEG (Moving Picture Expert Group) is appropriate. However, these audio coding schemes have a wide frequency band of a signal to be coded, which results in a problem that the bit rate of a coded signal increases.
Examples of conventional coding technologies include a technology of coding a signal with a wide frequency band at a low bit rate (e.g., see Patent Document 1). According to this, an input signal is divided into a signal of a low-frequency domain and a signal of a high-frequency domain, the spectrum of the signal of the high-frequency domain is replaced by the spectrum of the signal of the low-frequency domain and coded, and the overall bit rate is thereby reduced.
FIG. 1A to FIG. 1D show an overview of the above described processing of replacing the spectrum of high-frequency domain by the spectrum of the low-frequency domain. This processing is originally intended to be performed in combination with coding processing, but for simplicity of explanation, a case where the above described processing is performed on an original signal will be explained as an example.
FIG. 1A shows a spectrum of an original signal whose frequency band is restricted to 0≦k<FH, FIG. 1B shows a spectrum of the signal restricted to 0≦k<FL (where, FL<FH), FIG. 1C shows a spectrum obtained by replacing a high-frequency domain (high-frequency band) by a low-frequency domain (low-frequency band) using the above described technology and FIG. 1D shows a spectrum obtained by shaping the replacing spectrum according to spectrum envelope information about the replaced spectrum. In these figures, the horizontal axis shows a frequency and the vertical axis shows intensity of a spectrum.
In this technology, a spectrum of the original signal whose frequency band is 0≦k<FH (FIG. 1A) is expressed using a low-frequency spectrum whose frequency band is 0≦k<FL (FIG. 1B). More specifically, the high-frequency spectrum (FL≦k<FH) is replaced by the low-frequency spectrum (0≦k<FL). As a result of this processing, the spectrum as shown in FIG. 1C is obtained. Here, for simplicity of explanations, a case with a relationship of FL=FH/2 will be explained as an example. According to information about a spectrum envelope of the original signal, the amplitude value of the spectrum in the high-frequency domain of the spectrum in FIG. 1C is adjusted and the spectrum as shown in FIG. 1D is obtained. This is the spectrum which is the spectrum obtained by estimating the original signal.    Patent Document 1: National Publication of International Patent Application No. 2001-521648 (pp. 15, FIG. 1, FIG. 2)